1. Field of the Invention
The present invention relates to an image processing apparatus for processing an image signal.
2. Related Background Art
In a conventional digital printer (bi-level printer), since an output is constituted by bi-level data corresponding to on/off of dots, input image data (8 bits) from an input system, e.g., a CCD, is bi-level processed by a dither method, and gradation of an original image is expressed by a matrix consisting of a plurality of pixels. However, in the dither method, a resolution is degraded if gradation is improved. For example, in order to reproduce 17 gray levels including white, a threshold value matrix consisting of 4.times.4 pixels is necessary, and in order to reproduce 65 gray levels, a threshold value matrix consisting of 8.times.8 pixels is necessary. Since an output image is represented in units of threshold value matrices, the resolution of a halftone image is degraded as the size of the threshold value matrix increases. When the systematic dither method is used, a threshold value is periodically repeated in units of threshold value matrices. For example, if a dot image is scanned, a difference between original image data and a threshold value periodically varies, and a low-frequency moire pattern is formed in an output image resulting in a degraded image quality. An averaging error minimization method, wherein an error in an output pixel against original image data is compensated for the surrounding pixels so as to satisfactorily reproduce gradation of the original image data, has been proposed. This method is effective for eliminating moire pattern. However, pixels are unnaturally connected, and a micro texture structure is formed, resulting in a poor reproduced image. This method is improved by a CAPIX method (National Convention Record of the Institute of Electronics and Communication Engineers of Japan, 1985, 5-212) having good character reproducibility. However, with this method, a micro texture structure is formed again. In the above-mentioned systematic dither method, an edge of the reproduced image, in particular, an inclined thin line, becomes a zigzag pattern.
In the bi-level methods, original image data is compressed at high density in view of data volume. For example, if input image data has 8 bits per pixel, bi-level image data has 1 bit per pixel, resulting in 1/8 compression. Therefore, an image memory, an electronic file system, and the like for accumulating image data need only have a small capacity, and this is advantageous for communication for transferring image data. Thus, the above-mentioned techniques are indispensable for image processing.
With recent improvements in a digital multi-level printer, an image having a higher quality than that obtainable with a conventional digital bi-level printer can be obtained. For this reason, image data which is once compressed to bi-level data by the conventional dither method must be recovered to multi-level data by multi-level processing.
As described above, various methods for outputting an image including a halftone image by a digital printer have been proposed. Such methods include, e.g., the dither method, a density pattern method, and the like. These methods have the following advantages:
(1) An image including a halftone image can be displayed using a bi-level display device.
(2) The hardware arrangement of an apparatus is simple.
(3) A satisfactory image quality can be obtained. For these advantages, the above methods are widely applied in various fields. More specifically, as shown in FIGS. 7A and 7B, pixels (input pixel data) 28 of an input image are assigned to components of a threshold value matrix 25, and black or white is determined in accordance with the relation between the magnitude of the input pixel data and threshold value. The obtained result is output to a display screen 26.
FIG. 7A is for explaining the dither method. One input pixel 28 is assigned to one component of the threshold value matrix 25. FIG. 7B is for explaining the density pattern method. One input pixel 28 is assigned to all the components of the threshold value matrix 25. More specifically, in the density pattern method, one pixel of an input image is displayed by a plurality of cells on the display screen 26.
The dither method and the density pattern method are not essentially different from each other, except that one input pixel is assigned to one or all the pixels of the threshold value matrix. Of course, an intermediate method therebetween is present. For example, a method in which one input pixel is assigned to a plurality of components (four (2.times.2) components in FIG. 7B) of the threshold value matrix, has been proposed. Therefore, no essential difference is present between the above methods, and the dither method, the density pattern method, and a intermediate method will be called a dither method.
In the dither methods, since an image output is produced as bi-level data, it is constituted only by white or black, i.e., "0" or "1", and it is impossible to control the density of an image. However, various image output apparatuses, e.g., a laser beam printer, an ink-jet printer, a CRT device, and the like, are used. In this case, these output apparatuses respectively have their unique output characteristics, and an output density may often vary using the same bi-level signal.
For example, in a laser beam printer, on/off operation of a laser beam onto a photosensitive drum is controlled so as to form an electrostatic latent image, and the latent image is developed, thereby obtaining a visible image. In this case, the potential of a latent image changes in accordance with an ON time of the laser beam. The ON time and the potential have a nonlinear relationship, and the potential and a developing density also have a nonlinear relationship. Since the laser beam printer alone poses such many variation factors, outputs from different output apparatuses vary widely, and an output reproduced image is often considerably different from an expected one using the same bi-level signal. In particular, this is prominent when an image data bi-level processing means is provided only to a controller (e.g., a host computer) outside the output apparatus, and a plurality of output apparatuses reproduce an image in response to the same bi-level signal from the controller. This applies not only to the bi-level image signal, but also to a multi-level signal, e.g., a four-level signal, received from the controller.